This invention relates to an electrosurgical generator.
A radio frequency generator for electrosurgery conventionally has an output isolation transformer coupled to output terminals of the apparatus via a series coupling capacitor the function of which is to prevent low frequency output components causing nerve stimulation in the patient being treated. In electrosurgery, the impedance presented to the output terminals of the generator tends to vary widely, to the extent that the output stage is rarely matched, leading to reflection of power back into the output stage with consequently poor efficiency and unwanted heat dissipation. In this context "matched" means matching of the output impedance of the output stage to the load impedance, i.e. Z.sub.out =Z.sub.load. This effect may be overcome to a large degree by arranging for the output frequency to vary according to the magnitude of the load resistance, as described in published British Patent Application No. 2214430A, the disclosure of which is incorporated in this specification by reference. This prior apparatus has an output stage consisting of a self-tuning radio frequency oscillator having a power MOSFET driving a resonant output network coupled to the output terminals of the apparatus. The network consists of a transformer, the secondary winding of which is tuned by a parallel capacitor, and by a series coupling capacitor connected between one end of the winding and one of the terminals. It will be appreciated that the frequency of oscillation varies with load impedance, and reflection of power back into the generator can be maintained at relatively low levels over a wide range of load impedances. This is especially beneficial for a battery powered generator having a limited power output capacity. Stray capacitances resulting from connection of the generator to the load have little or no effect on matching. The prior disclosed apparatus does, however, have disadvantages. One of these is that under extreme load conditions, the only limit to the energy build up in the resonant output circuit is the Q of the circuit itself. If the generator is designed to produce a constant power output, for example, by pulsing the oscillator with a variable mark-to-space ratio according to sensed current drain and supply voltage, under extreme load conditions the oscillator energy at the required constant power is dissipated mainly by the resonant output circuit. Consequently, the output stage needs to be capable of dissipating the maximum design output power and the main oscillator device must be capable of withstanding the voltage levels necessary to achieve such power. Indeed, in bipolar electrosurgery the tissue impedance tends to be between 10.OMEGA. and 1K.OMEGA., while in monopolar application it tends to be between 100.OMEGA. and 10K.OMEGA.. Since power, P=V.sup.2 /R at resonance, the voltage applied to the output device can be .sqroot.(PR), P being the design power of the generator. In a low power generator (having a power output of 10 W typically), this may present no particular problems. However, for increased power outputs (for instance, up to 300 W) the resonant output circuit must be of heavy construction, and special measures must be taken to protect the output device.
Another difficulty concerns control of the oscillator device, specifically the feedback applied to a control terminal of the device to sustain oscillation. At higher power levels such control cannot be linear in the sense of operating the device as an amplifier so that the output current is proportional to the input voltage or input current, since the device must then dissipate a proportion of the oscillator power. The oscillator device is therefore preferably switched between the fully "on" and fully "off" conditions. Such switching brings the problem of higher harmonics being produced by the oscillator device, which are shunted by the resonant output circuit. This can, to some extent, be alleviated by switching the device 90.degree. phase-advanced with respect to the resonant output waveform. Nevertheless, in an extreme load condition, which gives very high Q in the resonant output circuit, the 90.degree. advance switching point causes the primary voltage to swing lower than the switching potential, assuming that the oscillator device is coupled between the transformer primary winding and ground. This occurs because the current injection immediately following the "on" instant is very high, the primary voltage at 90.degree. advance usually being higher than the switching supply voltage. With constant power feedback control, this condition is also an unstable one, since as more energy is injected into the resonant output circuit, the primary voltage at the switching-on point becomes progressively higher and therefore the peak switching current also becomes higher. This can lead to reverse biasing of the oscillator device, leading to inefficiency and sometimes to damage of the device. When the device is a power MOSFET, this reverse current flow causes forward biasing of the intrinsic diode which, being a comparatively slow device, then dissipates a significant level of power at radio frequencies. Whilst the reverse bias performance can be improved by coupling an additional fast recovery diode in parallel with the intrinsic diode, any conduction as a result of the transformer primary voltage falling below the switching voltage is inefficient.
It is an object of this invention to provide a generator of improved efficiency.